These days, there is increased the need of communication service in which broad band signals such as image data are transmitted with a predetermined quality. A band of the signals ranges from several MHz to several tens MHz so that there is widely adopted an optical transmitting system in which an electric-optic converter is used in a transmitter, an optical fiber having advantages such as small diameter, broad band, low loss, anti-electromagnetic induction etc. as compared to a coaxial cable is provided for propagating optical signal, and an optic-electric converter is used in a receiver. In such a system, a plurality of electric signals having different frequencies are converted in a plurality of electric-optic converters of a transmitter into a plurality of optical signals having different wavelengths which are propagated through an optical fiber, and the plurality of optical signals thus propagated are divided in a divider of a receiver by respective wavelengths and then converted in a plurality of optic-electric converters thereof into the plurality of electric signals whereby the plurality of electric signals can be transmitted in an optical wavelength division multiplexing transmitting system.
In such an optical wavelength division multiplexing transmitting system, there is used a transmission line change-over means in which an predetermined wavelength of optical signal is selected from wavelength division multiplexing signals propagated through an optical fiber connecting between a transmitter and a receiver. In the transmission line change-over means, a wavelength of a selected optical signal must be discriminated from others so that a correct wavelength of an optical signal is selected whereby an optical signal of a wavelength other than the predetermined wavelength is avoided to be selected into a selected one of divided transmission lines.
Even in an optical wavelength division switching device, there is used a wavelength tunable filter through which a predetermined wavelength of an optical signal is passed to be selected among wavelength division multiplexing signals. For the purpose, a wavelength of the optical signal thus passed through the filter must be discriminated from others.
Such an apparatus for discriminating an optical signal from others is described on pages 33 to 39 of "International Zurich Seminar on Digital Communication, Vol. B1.1, 1986".
In the apparatus for discriminating an optical signal from others, input data A and a discriminating signal A are multiplexed to produce a multiplexed signal A by a time-division multiplexer, and input data B and a discriminating signal B are multiplexed to produce a multiplexed signal B by another time-division multiplexer. The multiplexed signals A and B are converted in corresponding electric-optic converters to optical signals A and B having wavelengths .lambda..sub.1 and .lambda..sub.2 respectively. The optical signals A and B are multiplexed to be propagated through an optical fiber by a wavelength multiplexer. The optical multiplexed signal thus propagated through the optical fiber is divided into two optical multiplexed signals by a divider. Each of the two optical multiplexed signals are supplied to a corresponding one of transmission line change-over means. In one of the transmission line change-over means, an optical signal of a predetermined wavelength .lambda..sub.1 or .lambda..sub.2 is selected to be supplied to a corresponding transmission line. For this purpose, an optical signal which is selected in a wavelength tunable filter is divided into two optical signals by a divider. One of two optical signals thus divided is converted in an optic-electric converter to an electric time-division signal including input data and a discriminating signal which is then supplied to a separating circuit and a frame synchronus circuit. In the separating circuit, the input data and the discriminating signal are separated from each other in accordance with a frame synchronous signal supplied from the frame synchronous circuit so that the discriminating signal is only supplied therefrom. The discriminating signal thus separated is discriminated in a filter control circuit to be determined whether or not it is the discriminating signal A in a case where the aforementioned predetermined wavelength is .lambda..sub.1. When the discriminating signal 1 is not the discriminating signal A, it is decided in the filter control circuit that an optical signal of a wavelength other than a wavelength .lambda..sub.1 is selected. Thereafter, the wavelength tunable filter is swept from a short wavelength to a long wavelength and vice versa by the filter control circuit. The sweep of the wavelength tunable filter is stopped to be continued when an output of the separating circuit coincides with the discriminating signal A.
Thus, optical signals of predetermined different wavelengths are selected in transmission line change-over means to be supplied to respective transmission lines.
In the apparatus for discriminating an optical signal from others, however, there is a disadvantage that a circuit construction is complicated and a consuming electric power is increased for the reason why an optic-electric converter, a separating circuit and a frame synchronous circuit provided in a transmission line change-over means must be responded in a high speed to input data in a case where a speed of the input data is increased because the input data and a discriminating signal is multiplexed in a time-division.
In the apparatus for discriminating an optical signal from others, there is used a wavelength tunable filter in which an optical signal of a predetermined wavelength is selected from a wavelength division multiplexing signal to be supplied to a transmission line as described before. Such a wavelength tunable filter is described as a distributed feedback laser device (DFB LD) in "Proceedings, No. 326 of The institute of Electronics Information and Communication Engineers". The DFB LD is a current control type of a wavelength filter in which an injecting current less than an oscillation threshold value is varied in its level so that a center wavelength thereof is changeable. However, a wavelength filter utilizing the DFB LD has a temperature-dependent property in which a center wavelength is fluctuated by approximately 1A.degree./C.degree. because the DFB LD is fabricated from InP system material. For this reason, the wavelength filter must be controlled to be tuned to an optical signal of a correct wavelength even if a temperature is fluctuated. Such a wavelength filter is used even in an optical wavelength division switching means in which a plurality of optical signals each having a predetermined different wavelength from others are switched.
A conventional apparatus for tuning an optical wavelength filter is described on pages 750 to 752 of "Electronics Letters, July 2, 1987 Vol. 23, No. 14".
In the apparatus for tuning an optical wavelength filter, a center wavelength of a wavelength filter is controlled in advance to be near a predetermined wavelength of an optical signal which will be selected correctly. Thereafter, an optical signal passed through the wavelength filter is converted into an electric signal which is then multiplied in a multiplier with a discriminating signal supplied from an optical wavelength multiplexing circuit. A multiplied output is supplied from the multiplier to a low-pass filter so that a direct current component thereof is passed therethrough to be supplied to a filter driving circuit. Current injected into the wavelength filter is controlled in accordance with the direct current component by the filter driving circuit so that a center wavelength of the wavelength filter is tuned to a predetermined wavelength of an optical signal to be selected correctly, although detailed explanations of operation described above will be made later.
According to the apparatus for tuning an optical wavelength filter, however, there is a disadvantage that a discriminating signal must be supplied from an optical wavelength multiplexing circuit to a multiplier by use of a transmission line. There is a further disadvantage that a center wavelength of a wavelength filter is controlled in advance to be near a predetermined wavelength of an optical signal which will be selected correctly in a case where the center wavelength thereof is different to some extent from a tuning wavelength because an output of a low-pass filter is only used for a control signal by which the center frequency thereof is tuned to a predetermined wavelength of an optical signal to be selected correctly.
Accordingly, it is an object of the invention to provide an apparatus for discriminating an optical signal from others in which a circuit construction can be avoided to be complicated and electric power is less consumed even if a speed of input data is much increased.
It is a further object of the invention to provide an apparatus for tuning an optical wavelength filter used in an apparatus for discriminating an optical signal from others in which a discriminating signal is not necessary to be transmitted from an optical wavelength multiplexing circuit to a multiplier thereby being multiplied with an electric signal converted from an optical signal passed through a wavelength filter.
It is a still further object of the invention to provide an apparatus for tuning an optical wavelength filter used in an apparatus for discriminating an optical signal from others in which a center wavelength of a wavelength filter is not necessary controlled in advance to be near a tuning wavelength thereof.
According to one feature of the invention, an apparatus for discriminating an optical signal from others comprises,
a plurality of adding means, each including an adder in which input data and a discriminating signal of a frequency lower than a signal band of said input data are added to produce an added signal,
a plurality of electric-optic converters, each converting said added signal into an optical signal of a predetermined wavelength different from others,
a wavelength multiplexer for multiplexing optical signals supplied from said plurality of electric-optic converters to produce an optical wavelength division multiplexing signal which is propagated through a wavelength multiplexing transmission line,
a divider for dividing said optical wavelength division multiplexing signal into a plurality of optical wavelength division multiplexing signals, and
a plurality of transmission line change-over means, each supplying an optical signal of a predetermined wavelength to a corresponding transmission line and including a wavelength filter for passing an optical signal of a wavelength selected from plural wavelengths therethrough, a circuit for detecting a discriminating signal included in said optical signal passed through said wavelength filter, and a circuit for controlling said wavelength filter in accordance with said discriminating signal detected in said circuit for detecting whereby a center wavelength of said wavelength filter is tuned to a predetermined tuning wavelength.
According to a further feature of the invention, an apparatus for tuning an optical wavelength filter used in an apparatus for discriminating an optical signal from others comprises,
a circuit for producing an optical wavelength division multiplexing signal which includes a plurality of optical signals each having a different wavelength from others, each of said optical signals being frequency-modulated by a first discriminating signal and being amplitude-modulated by a second discriminating signal having a phase information of said first discriminating signal,
an optical wavelength tunable filter for passing an optical signal of a predetermined wavelength therethrough,
means for converting said optical signal passed through said optical wavelength tunable filter into an electric signal,
first and second filters for passing first and second signals of the same frequencies as said first and second discriminating signals,
a circuit for producing a third signal of the same frequency as said first discriminating signal in accordance with said second signal,
a circuit for producing a direct current component in accordance with a multiplication of said first and third signals, and
a circuit for controlling said wavelength tunable filter to be tuned to an optical signal of a predetermined wavelength by sweeping a center wavelength thereof in accordance with said direct current component.
According to a still further feature of the invention, an apparatus for tuning an optical wave length filter used in an apparatus for discriminating an optical signal from others comprises,
a circuit for producing an optical wavelength division multiplexing signal which includes a plurality of optical signals each having a different wavelength from others, each of said optical signals being frequency-modulated by a discriminating signal,
an optical wavelength tunable filter for passing an optical signal of a predetermined wavelength therethrough,
means for converting said optical signal passed through said optical wavelength tunable filter into an electric signal,
a circuit for producing a direct current component in accordance with a multiplication of said electric signal and said discriminating signal,
a circuit for producing a control signal in accordance with a frequency difference between said electric signal and said discriminating signal, and
a circuit for controlling said wavelength tunable filter to be tuned to an optical signal of a predetermined wavelength by sweeping a center wavelength thereof in accordance with said direct component and said control signal.